The present invention relates to the field of turbomachines, especially aeronautical turbomachines, and is intended for the repair of moving bladed discs.
To meet the increased performance requirements of engines, one-piece bladed discs or wheels, called “blisks”, are now manufactured from titanium alloy for the compressors of turbojet engines. In a conventional rotor, the blades are retained by their root, which is fitted into a housing made on the rim of the disc. The discs and blades are therefore manufactured separately before being assembled into a bladed rotor. In a blisk, the blades and the disc are machined directly from a forged blank—they form a single part. This technique permits substantial savings in the total weight of the engine to be made, but also substantial reductions in manufacturing costs. Fretting problems on the blade routes and cavities in service no longer exist.
However, this type of rotor has the drawback of being difficult to repair. In operation, the compressor blades may undergo damage due to impacts caused by the ingestion, via the engine, of foreign bodies or else due to erosion caused by dust and other particles entrained by the air flowing through the engine and coming into contact with the surface of the blades. This wear or damage, if it cannot be repaired according to the criteria specified in the manufacturer's documentation, involves replacing one or more defective blades. In the case of one-piece bladed components, the blades are integral parts of a massive component and, unlike in conventional arrangements, they cannot be replaced or even removed in order to be repaired individually. It is necessary to repair the part directly on the disc. The repair must therefore take into account all aspects of the component, with its size, its weight and, in the case of large components, accessibility to the zones to be repaired.
Thus, in the case of a blisk, the regions generally concerned by repair are, for each blade, the tip, the aerofoil corner on the leading edge side, the aerofoil corner on the trailing edge side, the leading edge and the trailing edge. The blades are of what is called a three-dimensional design, with the particular feature of having a shape that changes in three directions and a variable thickness along this changing profile. The maximum extent of the zones able to be repaired takes into account the use of the engines and of the aerodynamic loads supported by the components.
The repair techniques that have been developed consist in removing the damaged region on the damaged blades and then in replacing the removed portion with a part of suitable shape, or else by build-up welding. These techniques generally employ a conventional machining operation, for removing the damaged portion, contactless inspection of the repaired part, ultrasonic peening and specific machining for re-work of the repaired zone.
The present invention relates to repair by build-up welding.
Repair is particularly difficult to carry out in the case of certain alloys used, the welding of which results in the formation of volume defects. This is especially so for the titanium alloy Ti17. This alloy is mentioned for example in the Applicant's patent application EP 1 340 832, which relates to a product, such as a blade, made of this material. When performing build-up welding, the TIG or microplasma techniques conventionally and widely used in the aeronautical industry do not allow titanium Ti17 to be treated under conditions allowing acceptable results to be achieved.
These conventional build-up welding techniques result in the formation of defects. Thus, TIG build-up welding, employing a substantial amount of energy compared with the small thickness involved, generates strains and leads to the formation of a large number of pores, such as micropores or microblisters, and also an extended heat-affected zone (HAZ). These micropores, which are not very easily detectable, generate a weakening in the mechanical properties by up to 80%. Such weakening in the behaviour of the components in operation is unacceptable, and this type of build-up welding cannot be applied. Microplasma build-up welding results in the formation of a smaller HAZ, but it is still relatively large. Furthermore, the method requires particular attention and a periodic inspection of the equipment and products used, so that no operating parameter of the machine drifts and modifies the expected results.
U.S. Pat. No. 6,568,077 describes a method of repairing a blade on a blisk in which the damaged portion of the blade is machined and then, in a first operating mode, the missing portion is built up by deposition of metal by means of a tungsten-electrode arc-welding (TIG) machine. In a second operating mode, an insert is welded by means of an electron-beam welding machine. The profile of the blade is then restored by appropriate machining. However, this method does not mention the problem encountered when welding certain titanium alloys.
The Applicant has found that the use of a build-up welding technique by application of a laser beam obviates the problems encountered by the usual techniques. In particular, laser build-up welding appears to be a technique that minimizes the defects in the weld zone.
Laser build-up welding is already known and used, for example in applications where metal contours have to be generated, especially from CAD data. The walls have a thickness of between 0.05 and 3 mm and the layers are 0.05 to 1 mm in height. The technique makes it possible to achieve excellent metallurgical bonding to the substrate.
The technique of build-up welding by means of a laser beam has the following advantages: the heat influx is constant over time. Heat has no time to accumulate within the volume and to diffuse—it follows that there is little outgassing in the case of titanium and a limited reduction in strength. Furthermore, the repeatability and reliability of this technique are good, once the machine parameters have been set, and it is easily controlled.
The laser techniques currently employed involve simultaneously adding filler material and radiating the substrate with the laser beam. The material is generally deposited in the work zone in the form of a powder or a metal wire. In other versions, it is sprayed in the form of powder jets into the work zone using a suitable nozzle.